1. Field of the Invention
The present invention relates to a processing apparatus employing a microwave plasma, as a plasma processing apparatus fabricating semiconductor elements and so forth employing a low temperature plasma. More particularly, the invention relates to a microwave plasma processing apparatus suitable for large area processing in the CVD, etching, sputtering methods and so forth.
2. Description of the Related Art
A plasma processing technology employing a microwave is important due to its capability of reducing contamination of electrode materials, because typically it employs an electrodeless discharge. Also, a microwave discharge using an electron-cyclotron-resonance (ECR) has attracted attention due to its excellent characteristics, such as (1) uniformity of directionality of motion of ions generated for a capability of discharge at a low pressure (10.sup.-5 Torr), (2) capability of generating a high density plasma, and (3) long life and capability of use of an active gas for an electrodeless discharge.
FIG. 17 shows a plasma chamber employing the conventional ECR discharge, and a basic construction of an ECR-CVD apparatus employing the plasma chamber. A microwave source, not illustrated, is formed by employing a magnetron of 2.45 GHz. Typically, a rectangular wave guide having a size of 96 mm.times.27 mm or 109 mm.times.54 mm is employed as a microwave guide 3. Also, the plasma chamber 1 has an internal diameter of approximately 200 mm. In many cases, the plasma chambers 1 employ a cavity resonance structure for microwaves for an effective charge of microwave power. The microwave power introduced into the plasma generation chamber 1 through the wave guide 3 is consumed for a generation of plasma.
The generated plasma is withdrawn toward a sample 9 through a plasma withdrawing window A. The plasma withdrawing window A is provided for maintaining the cavity resonance structure of the plasma generation chamber 1 and for withdrawing the uniform portion of the generated plasma. Namely, since an electric field strength of the microwave is weakened at the peripheral portion of the plasma generation chamber 1, to thus cause a difference in the plasma densities at the center portion and at peripheral portions, the plasma withdrawing window is designed to withdraw only a uniform portion of the generated plasma. A magnetic coil 4 is provided for establishing a direct current magnetic field satisfying an ECR condition (875 Gauss), and the generated plasma is effectively dispersed toward the sample 9 in a weak magnetic field region. As set forth above, by radiating the plasma onto the sample, a process, such as CVD, etching or so forth can be performed for a substrate.
In the conventional microwave plasma processing apparatus having a construction by which the microwave power generated by a magnetron is introduced, as set forth above, into the plasma chamber 1 through the wave guide 3, a propagation mode of the microwave in the rectangular waveguide is typically TE10 mode. Where the plasma chamber has a rectangular configuration, the mode of the microwave is a rectangular TE10 mode as illustrated in FIG. 18. Conversely, when the plasma generation chamber has a cylindrical configuration, the mode of the microwave becomes the TE11 mode as illustrated in FIG. 19. It should be noted that, in FIGS. 18 and 19, (a) are sections respectively taken on a plane perpendicular to the propagation direction of the microwave, and (b) are sections taken on a plane parallel to the propagation direction of the microwave. It is to be further noted that the solid lines represent electric line of force and broken lines represent magnetic force lines.
As a technology enabling a generation of a greater area of plasma, there is known a technology for controlling a mode of the microwave, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-122123. As shown in FIG. 20, this technology employs a mode-converting wave guide having a quadrilateral configuration incorporating a pair of mutually opposed arc-shaped segments arranged in a circle centered at an axis of the plasma chamber, for controlling the mode of the microwave. With the disclosed prior art, it is possible to form the TM11 mode and TM10 mode as shown in FIGS. 21 and 22.
Nevertheless, the plasma generation technology employing the microwaves in the TM01 mode and TM11 mode disclosed in Japanese Unexamined Patent Publication No. 1-122123 encounters the following problems. With respect to the TE11 mode, TM11 mode and TM10 mode, a radius a of a circular wave guide has the following relationship, taking the cut-off wavelength of a propagating microwave as .lambda.c: EQU .lambda.c=3.412a (TE11 mode) EQU .lambda.c=1.640a (TM11 mode) EQU .lambda.c=2.613a (TM01 mode)
As can be appreciated from these equations, in the TM11 mode, the radius a of the cylindrical wave guide can be 2.08 times that in the TE11 mode, but in TM11 mode, as shown in FIG. 21, an electromagnetic field distribution of the microwave is not uniform, and thus difficulties arise in the generation of a uniform plasma. On the other hand, as shown in FIG. 22, although a uniform electromagnetic field of the microwave can be established in the TM01 mode, an expansion of the area to be obtained is merely 1.31 times that of the conventional TE11 mode.